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POUse python dict to lookup mutable objects
 

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    <p>I have a bunch of <code>File</code> objects, and a bunch of <code>Folder</code> objects. Each folder has a <code>list</code> of files. Now, sometimes I'd like to lookup which folder a certain file is in. I don't want to traverse over all folders and files, so I create a lookup <code>dict</code> file -> folder.</p> <pre><code>folder = Folder() myfile = File() folder_lookup = {} # This is pseudocode, I don't actually reach into the Folder # object, but have an appropriate method folder.files.append(myfile) folder_lookup[myfile] = folder </code></pre> <p>Now, the problem is, the files are mutable objects. My application is built around the fact. I change properites on them, and the GUI is notified and updated accordingly. Of course you can't put mutable objects in dicts. So what I tried first is to generate a hash based on the current content, basically:</p> <pre><code>def __hash__(self): return hash((self.title, ...)) </code></pre> <p>This didn't work of course, because when the object's contents changed its hash (and thus its identity) changed, and everything got messed up. What I need is an object that keeps its identity, although its contents change. I tried various things, like making <code>__hash__</code> return <code>id(self)</code>, overriding <code>__eq__</code>, and so on, but never found a satisfying solution. One complication is that the whole construction should be pickelable, so that means I'd have to store <code>id</code> on creation, since it could change when pickling, I guess.</p> <p>So I basically want to use the identity of an object (not its state) to quickly look up data related to the object. I've actually found a really nice pythonic workaround for my problem, which I might post shortly, but I'd like to see if someone else comes up with a solution. </p>
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    1. COAn attribute of the file object telling you in which folder they are is not an option?
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    2. COAgreed with Jblasco. Try to have a unique attribute every time you create a new File object
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    3. CO@Jblasco: Yeah, that is the dead-simple pythonic workaround I went with :-). However, I'm still wondering if there is a way to do it with an external dict. One reason: There might be different classes of folders (directories, tags, smart searches). I'd have to add a special attribute for each kind.
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